Thermosetting polymer materials are often used as adhesives and composite resins in applications that involve exposure to elevated temperatures. Therefore, there is an on-going need to improve thermo-chemical stability of cured resins in environments involving both elevated temperature and exposure to chemical agents (such as oxygen, water vapor, and acid vapor). This on-going need has given rise to a number of alternative materials that involve the cure of reactive groups such as cyanate esters, maleimides, benzoxazines, phthalonitriles, and aryl ethynyls. While these conventional resins are valued for their ease of use in affordable processes, such as resin transfer molding and filament winding, and for offering maximum service temperatures that are higher than those afforded by epoxy resins with similar processing characteristics, deficiencies remain.
For example, for conventional cyanate esters having more than about 3 mmol cyanurate per cubic centimeter, the fraction of uncured reactive groups is often larger than about 5% and, in fact, may be as much as 20%. These uncured reactive groups become detrimental to the performance of the thermoset polymer, e.g., unreactive cyanate esters may react with water at elevated temperatures to release carbon dioxide gas, which leads to blistering and mechanical failure of the resultant resin.
Some conventional approaches to combat the issues with uncured reactive groups has been to increase the final cure temperature and/or to increase catalyst level and activity; however, the former often requires temperatures exceeding 300° C., which impose significant added costs to many processing techniques while the latter is associated with decreased thermal stability in wet environments. Still another approach adds alkyl groups and thereby decreases the thermo-chemical and thermo-oxidative stability of the resin. In fact, the decreased thermo-chemical stability limits the maximum use temperature of such resins to values at least 50° C. lower than would otherwise be possible. Furthermore, the substantial molecular volume increase associated with such alkyl chains decreases the density of cross-linkages within a network segment and lowers the glass transition temperature of the resin.
There remains a need for high-temperature thermosetting resins having a glass transition temperature exceeding 280° C., with a flexible moiety having a high level of thermo-oxidative resistance, and which minimally adds to the molar volume of the network into which it is incorporated.